1/14 + 1/35 + 1/65+ .....+ 2/ x^2 + 3x = 1/9
⇔2/28+2/70+2/130+...2/x(x+3)=1/9

⇔ 2/3(3/(4.7)+3/(7.10)+...+3/(x(x+3)))=1/9

⇔ 3/(4.7)+3/(7.10)+...+3/(x(x+3))=1/6:3/2=1/6

⇔1/4-1/7+1/7-1/10+...+1/x-1/(x+3)=1/6

⇔1/4 - 1/(x+3)=1/6

⇔1/4-1/6=1/(x+3)

⇔1/12=1/(x+3)

⇔12=x+3